This invention relates to a method of measurement and closed loop control in vacuum, reduced and overpressure furnace operations by measurement of actual part outgassing as an indicator of the actual state of the parts. With this information corrections can be made automatically as required via microprocessor control of the extrinsic conditions in a feedback loop.
Powder metallurgical operations and the like in the vacuum environment are often affected by the residual chemical atmosphere created by the outgassing from both the furnace and the parts. The atmosphere itself gives valuable intrinsic information about the state of the parts being processed through their outgassing even if the actual furnace atmosphere chemistry is unimportant in affecting part processing. The effect in terms of the atmosphere oxidizing/reducing character, or the carburizing/decarburizing character, as examples of this type of information, is usually dependent upon ratios of residual gas species rather than upon the absolute magnitude of the individual partial pressures alone. Specifically, for example, the hydrogran to water and the carbon monoxide to carbon dioxide ratios are important in determining the oxidizing/reducing character, as is well known. In another example, methane divided by the square of the hydrogen concentration is significant in determining the carburizing/decarburizing potential of the furnace in certain systems such as tungsten carbide. For this reason measurement of the percent composition of the residual gases in reduced pressure metallurgy and ceramic processes is generally sufficient for control and characterization purposes. This has advantages in minimizing absolute calibration common mode errors of the spectrum of gases measured.
In the case of carbon control, best known approaches to atmosphere adjustment in a vacuum sintering furnace consist of either admitting a predetermined gaseous atmosphere such as hydrogen in particular segments of the sinter batch cycle or in deliberately allowing the furnace to achieve a given degree of chemical contamination. As an example of this later approach, sometimes a deliberate excess of binder material is added to the furance; or periodically the furnace is heated up without parts to burn off excess contamination; or even more periodically rebuilt in terms of the internal insulation and exposed surfaces. All of these approaches are used depending upon the degree of carburizing and decarburizing that is desired for a given product.
In metallurgical or ceramic systems where carbon control is not the issue, control may depend upon following an evolution schedule such as determining that a particular gas percentage composition or ratio is depleted or increased in some functionally appropriate fashion. These control attributes of the batch cycle extrinsic conditions in turn will depend upon the final properties desired and the particular type of powder being used for a given product as determined by analytical assessment before part pressing and also by experience of post hoc metallurgical examination of recent parts from a given process. Other technologies require batch cycles with temperature profiles so as to completely degas the parts in accord with predetermined temperature regimes, for a given gas species which may not be the most significant contributor to the total pressure and thus are unobserved with conventional instrumentation. Therefore a situation can result where nominal conditions are, of necessity, presumed (incorrectly) to apply to all production heats.
All of these prior approaches have in common the feature that in addition to their lack of precise control, none depend upon in situ measurement of the real time furnace/part conditions. Furthermore, except for deliberately admitting a predetermined gas quantity, none have any specificity with respect to differing sinter cycle segements which themselves may require differing chemical atmospheres. For example, the delube portion of the cycle for tungsten carbide normally benefits by a reduction of the lubricant remnants and surface oxides while the remaining segments normally are ideally more neutral. It is one object of this invention to provide a generalized control algorithm feature which will enable an operator to easily program an appropriate control mode for a given process based upon intrinsic measurements for the part/furnace atmospheric constituents.
A prior art attempt to control a batch operation in a heat processing furnace is shown in U.S. Pat. No. 4,288,062 to Gupta et al. In this patent, a system is described for the monitoring and control of the carbon content of the atmosphere of a furnace such as a one atmosphere carburizing heat processing furnace for steel parts. This patent suggests the use of an infrared analyzer for the measurement of absorbed CO and CO.sub.2. It is to be noted however that certain components of the atmosphere of a furnace such as a vacuum furnace which would be useful in control of such furnace cannot be measured according to the Gupta disclosure and inasmuch as important measurable gases such as hydrogen and nitrogen are wholly or substantially invisible to infrared. Furthermore, obvious problems exist with regard to vacuum pressure levels in using the infrared approach. Thus, outside the constant one atmosphere surface carburizing environment, the Gupta patent does not disclose a method of controlling metallurgical processes as for example in a vacuum furnace. While Gupta et al suggests that other means of analysis might be used, it is not indicative that such analysis could be used on gases other than CO and CO.sub.2 nor how the variable total pressure conditions as in vacuum metallurgy could be handled for quantitative measurement. Besides the foregoing, a major distinction between the approach outlined in this invention and that of Gupta et al is that in a vacuum, or a reduced pressure environment, sensitive real time general techniques like mass spectrometry have the ability to measure intrinsic outgassing effects and thus the actual state of the parts, rather than just the carbon potential or other characteristics of an extrinsic environment provided for them.
A good example of this intrinsic measurement ability of real time part properties and a further object of this invention is the determination of pore closure. In certain powder metallurgy and ceramic operations, the determination of the actual time of sinter pore closure and thus densification is of great importance in part processing. According to this aspect of the invention, such determination is accomplished by measuring the outgassing rate of gas which is principally, although not limited to, nitrogen from the internal pores of the sintered parts as it undergoes densification. The resulting output curve, obtained by monitoring the furnace partial pressure of the gas in question, shows the effect of such outgassing being attenuated since the pore closure decreases the conductance of such gas from the sintered part. The novel aspect of this invention is that it makes use of an existing physical phenomenon: the presence of gas in the incompletely sintered pores to produce a signal proportional to the degree of pore closure since this gas partial pressure can be measured in the furnace after its conductance through the sintered part. By observation of such a curve in real time, the current degree of sinter densification can be determined since densification corresponds to pore shrinkage which, in turn, attenuates the conductance of the internal gas from the sintered part into the furnace. The structure of the attenuation curve also reveals information as to the spatial homogeneity of the densification process over the furnace load. If temperature is not uniform within the hot zone, each part densifies corresponding to its own temperature which in turn superimposes the gas evolution attenuation effect for the net gas signal measured. This has a characteristic profile which can be deconvolved to provide valuable information on the real time processing homogeneity.
The classical problem in such sintering measurements is that thermometry can be, and usually is, as much as hundreds of degrees off the true temperature of the furnace parts. Thus, great difficulty is encountered in correctly implementing, in time sequence, the proper sintering events based on measurements of temperature. For example, in some cases subsequent efforts following pore closure at higher temperature to prevent liquid phase species removal by evaporation involve allowing the furnace pressure to rise. If this is allowed to happen prior to complete pore closure (densification), the result is incomplete sinter part degassing. Since the relationship between the temperature, as read by the thermocouple system, and the furnace part load varies with many factors such as the current size of the load (thermal mass), great inaccuracy can result leading to a seriously uncontrolled process.
Best known prior art approaches to measurement of part densification in real time involve the use of dilatometry which is a laboratory tool not amenable to use in the production environment. Otherwise, the sinter process relies on temperature measurement to determine the state of the sinter cycle at a given time. As we have described above, this is very unrealiable and, in fact, the relationship between the temperature of a measurement thermocouple and a true temperature of the furnace parts can vary widely from run to run. Thus, the process invention described here provides the first useful production method to make use of intrinsic in situ information.